The present invention relates to a process for preparing a printing plate and a printing plate produced by said process. More particularly, the present invention relates to a process for preparing a printing plate using an N-type photoconductor layer having an optical memory property and a printing plate produced using the printing plate preparation process.
Many processes for preparing a printing plate have hitherto been proposed in the art. Among them, a process commonly used in the art comprises coating a photosensitive resin on an aluminum plate or a zinc plate, bringing the coated plate into close contact with a photographic original plate having a desired pattern, and conducting exposure and development. In recent years, a plate product of an aluminum plate bearing thereon a previously coated photosensitive resin has become commercially available as a PS (presensitized plate), and the exposure in a desired pattern of the PS plate followed by development to prepare a printing plate has spread in the art.
On the other hand, in order to prepare a printing plate in a simpler manner, an electrophotographic plate preparation process, which can simply prepare an ink-receptive toner image on the printing plate, has been developed wherein the surface of a photoconductor or an organic photoconductor substrate is subjected to corona discharge treatment in a dark place and then exposure and toner development (dry powder development, wet liquid development, or the like) to form a toner image on the surface of the photoconductor substrate.
In an electrophotographic plate preparation process utilizing the properties of an oxide photoconductor, particularly zinc oxide (ZnO), photoconductor substrate prepared by coating the surface of a conductive substrate with a coating prepared by dispersing a fine powder of ZnO in a polymer binder and then drying the resultant coating is subjected to a series of the above steps of corona discharge, exposure, and toner development to prepare a printing plate. This process is advantageous in that all the plate preparation operations can be carried out in a dry state. Since ZnO is inherently hydrophilic, it has good wettability by dampening water used in offset printing, rendering the printing plate prepared by the above process suitable for offset printing.
On the other hand, in an electrophotographic plate preparation process using an organic photoconductor (OPC), the exposure and development are carried out in the same manner as in the case where ZnO is used. Since, however, OPC is fundamentally hydrophobic, OPC cannot receive dampening water used in offset printing when an OPC layer is present on the printing plate. In this case, not only a toner image area but also the OPC layer as a non-image area unfavorably receives an ink, making it impossible to selectively form an ink image area. For this reason, after the formation of a toner image on the surface of a substrate, the OPC layer of the non-image area should be dissolved away to expose the hydrophilic surface of the substrate.
The currently most popular PS plate has gained public favor in that the resolution and durability are excellent and good print quality can be attained. The PS plate, however, is disadvantageous in that it is expensive and has unsatisfactory photosensitivity (low sensitivity) in a novel exposing method, wherein optical drawing is carried out at a high speed, is not always compatible with the digital printing plate preparation process being in the course of development in recent years. Efforts have, of course, been made to increase the sensitivity of the photosensitive resin used in the PS plate to such an extent that the PS plate becomes suitable for laser digital plate preparation. The sensitivity, however, could not have been increased to that of the electrophotographic printing plate preparation process.
A printing plate prepared by an electrophotographic plate preparation process utilizing an oxide photoconductor can be prepared at low cost by using a conductive paper substrate or the like and has been used extensively in the field of simple printing by taking advantage of dry developer. In general, however, this printing plate has low resolution and cannot be applied to high-quality printing and printing in large volumes. The use of liquid developer can improve the resolution as compared with the use of dry developer. Since, however, the plate wear is substantially the same, the use in printing in large volumes is impossible.
The printing plate prepared by the above electrophotographic printing plate preparation process utilizing the organic photoconductor (OPC) has a plate wear of 100000 to 200000 sheets. In this printing plate, however, as described above, the OPC layer at the non-image area should be dissolved away with the toner image being used as an etching resist to expose the hydrophilic surface of the substrate. This operation deteriorates the high resolution attained by the wet development, and the resultant printing plate generally has such a resolution as will be applicable to printing for newspaper (100 to 133 lines/in.) but could not have been put to practical use for high-quality printing (not less than 175 to 300 lines/in.).